1. Field of the Invention
This invention relates to apparatus, and the corresponding method for visible light curing of dental targets such as restorative materials placed in teeth, and in particular an apparatus and method for providing and using a relatively low divergence collimated beam of light which is non-contact applied to the dental target.
2. Background of the Invention
In the field of dentistry, photo-curable compositions have been used to produce dental sealents, dental filling materials, dental adhesives and the like. In a first generation of such applications, the restorative materials were curable by the application of ultraviolet radiation. In order to provide such UV radiation, dental guns and other forms of apparatus for producing concentrated beams of UV radiation were provided. See, for example, the prior U.S. Patents to Gonser, U.S. Pat. Nos. 4,112,335 and 4,229,658, assigned to the same assignee. However for many applications, including the dental area, it was found that visible light curable compositions were preferred. Visible light is more efficient in crossing the boundary between dissimilar materials, through normal tooth structure, and through materials which have already been cured. Accordingly, apparatus for producing concentrated visible light was produced and made available for dental applications. See the Gonser U.S. Pat. No. 4,385,344, "Visible Light Apparatus For Curing Photo-Curable Compositions", assigned to the same assignee.
The use of visible light curing techniques in the dental field has become relatively widespread, but at the same time certain limitations have been observed. The apparatus that has been available in the art produces a light output which is relatively divergent, e.g. the beam may diverge at about 30.degree. from the beam axis. It is known from the physical laws that light is directed at tooth structure will scatter as a function of the angle of incidence. The greater the divergence of the light flux from a path directly normal to the tooth surface, the greater the degree of scatter, and thus reduced penetration into the tooth structure. In prior art apparatus, a great deal of the light output is found to scatter, such that is does not penetrate into the light sensitive material placed in a tooth cavity. This characteristic of prior art light sources leads to relative inefficiency and unreliability for photocuring restorative materials that are either thick (&gt;2 mm) or located behind natural tooth structure, such as in situations where the light cannot be directly applied to light sensitive materials.
Another feature of prior art light sources, particularly for dental applications, is that they provide very condensed small area light outputs. Since the light diverges at a great angle, it is necessary for the operator to position the output directly on the light sensitive restorative material, i.e. the tip end of the light output must make contact with the restorative material or the tooth structure. As a result of this requirement, the procedure of photocuring has been very operator dependent, since the dentist or dental technician must position the light precisely at the proper position. If the target area to be cured presents a cross section which is substantially larger then the cross section of the light output, then the operator must take great care to manipulate the contact point or light output aperture so as to achieve light penetration into all applicable areas where all of the restorative material must be photocured. Thus, it has been seen that there are instances where incomplete curing has resulted. Undercured zones of light polymerizable material can be caused by such insufficient light application. In these zones there is a depletion of free radicals where the light has penetrated to only low intensities, such as around the periphery of the light beam. When, at a later moment in time, a light beam of high intensity is moved into position to penetrate these peripheral locations, the source for the production of additional free radicals is limited due to the previous depletion.
There has thus developed a substantial need in the art for a technique and apparatus to overcome the inherent problems with the presently available hardware for photocuring in dental applications. Thus, the typical oral cavity photocure light device which is presently available, has a drop in light flux of 80%, or to 20% of its output at only 1 cm distance, due to the high light ray divergence characteristic of the light beam. Most present photocure light beams must be applied essentially at contact, or not greater than about 2 mm distance from the tooth restorative material, in order to achieve photocuring suitable for dental restorations. The beam sizes typically range between 5 mm and 8 mm in diameter, delivered from fiber optic or quartz rods. In applying such a beam to larger area restorations, such as posterior or cosmetic applications, it is required that the light guide be manually moved near the surface by the operator and, therefore, the end result is a method which is operator technique dependent. Even larger area applicators, which produce 10 mm diameter light beams from optical fibers or the like, require "contact" photocuring, which is extremely cumbersome due to the large size of the applicator which must be applied in the patient's mouth. Larger size light beams could be made available by simply enlarging the dimensions of present devices, but these would be expensive, operationally inefficient and unacceptably heavy and cumbersome. For example, a 10 mm diameter fiber optic light guide is an impractical approach, and would not be an acceptable choice for most dental applications.